Cerebrospinal fluid

A type of transcellular fluid

Function

1. Act as cushion
   --> Protection from injury
2. Water bath effect
   --> Brain effectively weights only 50g

NB:

• No lymphatic channels in brain
• CSF fills the role of returning interstitial fluid and protein back into circulation

Location

In the ventricles and subarachnoid space, around the brain and spinal cord

Within the meninges

Quantity

Total volume: 150mLs

Daily production: 550 mLs/day
--> turnover of 3-4 times/day

Circulation

• Formed by
  * Choroid plexus (50-70%)
  * Ependyma of the walls of the ventricles (30-50%)
• CSF flows through the foramens of Magendie & Luschka
  --> Into the subarachnoid space of the brain and spinal cord
• Absorption by
  * Arachnoid villi (90%)
  * Directly into cerebral venules (10%)

Relationship with ICP

• Normal ICP: 5 - 15 mmHg
• Rate of CSF formation
  --> Constant, not affected by ICP
• Absorption of CSF
  --> Increases linearly as ICP increases above 68 mmCSF
• At a pressure about 11 cmH2O (or 112mmCSF, 8mmHg)
  --> Rate of secretion = absorption

NB:

• 1 mmHg = 1.36 cmH2O
• 1 cmH2O = 0.735mmHg

Composition of CSF

• Identical to brain ECF
• Different from plasma

Difference between CSF and plasma

[WG21:p616]

In CSF

• pCO2 is higher (50mmHg)
  --> Lower CSF pH (7.33)
• Protein content low
  * 0.2g/L (compare to 60g/L in plasma)
  --> Low buffering capacity
• Glucose concentration is lower
  * 64% of plasma level
• Cholesterol content is very low
  * 0.1% of plasma level
• Chloride concentration is slightly higher
  * 113 meq/kg H2O
• Potassium concentration is lower
  * 2.9 meq/kg H2O or 60% of plasma level
• Bicarbonate level is about the same
• [Ca2+] is lower
• [Mg2+] is higher

NB:

• Na+ is actively transported into CSF at the apical membrane of ependymal cells
• Ependymal cells also contain carbonic anhydrase and produce HCO3- into CSF to provide buffering